Patent Application
20060013942
1. Field of the Invention
This invention relates to the art of treating metals, metal alloys and metal oxides, and more particularly to a new and improved method for enhancing the electrical conductivity of metals, metal alloys and metal oxides.
One area of use of the present invention is in the manufacturing of electrodes for capacitors, batteries and the like, although the principles of the present invention can be variously applied. Metals and metal alloys have a native oxide present on the surface. This is an insulating layer and, hence, if the material is to be used as a substrate for an electrode, the oxide has to be removed or made electrically conductive.
If the oxide is removed by chemical treatment, such as by etching with an acid or electrolytic etching to expose the underlying metal, special steps must be taken in order to complete the electrical contacts before the native oxide can be regenerated and interfere with the electrical contacts. Such measures require special apparatus and extremely careful handling of the materials. All of this adds cost to fabrication of these materials into electrical devices to which electrical contact must be made.
Another approach involves removing the oxide layer and plating the bare substrate metal with an expensive noble metal, such as silver, gold, or alloys of silver, gold and platinum, or the formation of an electrically conducting compound on the bare substrate surface. The overcoating materials are expensive and the steps required to plate the substrate are costly and time consuming. In addition, the metal plating or electrically conductive compound must be disposed on the substrate as a continuous film for maximum performance. Therefore, the plating or compound formation typically is carried out after the substrate metal is formed into its final shape for the intended electrical device in order to avoid damage to the coating. This, in turn, adds to the cost and complexity of the manufacturing process.
2. Prior Art
U.S. Pat. No. 5,098,485 to Evans proposes a solution to the oxide problem by altering the native oxide from an electrically insulating to an electrically conducting condition. This is done without removal of the native oxide layer to expose the underlying metal or alloy. A solution containing ions of an electrical material is applied to the native oxide layer, and then the substrate, oxide and applied ions are heated to an elevated temperature for a time sufficient to incorporate the ions into the oxide layer, changing it from an electrical insulator to an electrical conductor.
U.S. Pat. No. 5,599,580 to Muffoletto et al. relates to a method for improving the electrical conductivity of a substrate by depositing a small amount of metal thereon and then directing a high-energy beam onto the substrate. This intermixes the deposited material with the native oxide of the metal substrate, changing the native oxide from being electrically insulating to electrically conductive. The high-energy beam is an ion beam from a high-energy ion source or a laser beam. The substrate with the electrically conductive native oxide layer is useable in the manufacture of electrodes for capacitors and batteries.
It would, however, be highly desirable to provide a new and improved method for enhancing the electrical conductivity of metals, metal alloys and metal oxides that does not require additional heat treatment, provides control over the density and depth of the material introduced to the treated surface, can be performed in a manner preventing substrate degradation and deformation, and improves the quality of the treated surface.
The present invention provides such a method for improving the electrical conductivity of a substrate of a metal, metal alloy or metal oxide by doping the native oxide with a suitable material causing the oxide to become electrically conductivity. This is done with ions embedded into the metal, metal alloy and/or metal oxide causing the oxide layer to become electrically conductive. The method can be used for metal, metal alloys and/or oxide and mixed oxides from Groups IVA, (Ti, Zr, and Hf), VA, (V, Nb, and Ta), VIA, (Cr, Mo, and W), Al, Mn, Ni, and Cu). The ions or mixtures of ions to be implanted are from Group VIIIA metals (Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt) elements. These ions are implanted using an ion implantation process. This includes elements from Group VIIIA being in the form of charged, energetic particles that are directed into the substrate metal, metal alloy and/or oxide from Groups IVA, VA, and VIA. The doped ions reside in the substrate as impurities.
The method of the present invention advantageously provides a doping method that does not require additional heat treatment and also provides control over the dose and depth of the ions implanted in the treated surface. The method can be performed at low temperatures preventing substrate degradation and deformation. It is believed that the quality of the treated surface is improved by the method of the present invention. An additional advantage of the present invention is that the surface can be treated to passivate it from chemical reaction while still providing adequate electrical conductivity. Stainless steels having native insulating oxides can also be treated to provide an electrically conductive oxide layer.
A substrate treated by the method of the present invention is ready for further processing in the manufacture of an electrode for use in a capacitor, a battery, and the like. Typically, in the case of a capacitor, an appropriate electrode material is deposited on the treated substrate surface by techniques well known to those skilled in the art. Examples of electrode materials are redox pseudo capacitance materials such as, but not limited to, oxides and mixed oxides of ruthenium, iridium, manganese, nickel, cobalt, tungsten, niobium, iron, molybdenum or under potential deposition systems such as palladium, platinum, lead dioxide or electro-active conducting polymers such as polyaniline, polypyrole and polythiophene.
The foregoing and additional advantages and characterizing features of the present invention will become clearly apparent upon a reading of the ensuing detailed description together with the included drawings wherein:
Metals and metal alloys have a native oxide present on the surface that is electrically insulating and must be removed or made electrically conductive if the metal or metal alloy is to be used as an electrode in devices such as capacitors and batteries. Referring to
In accordance with the present invention, the native oxide layer 12 is doped with a suitable material causing it to become electrically conductive. Doping is by ions embedded therein. The ions or mixtures of ions to be implanted are from Group VIIIA (Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt) elements. These ions are implanted using an ion implantation process such as those used in the manufacture of semiconductors. In this process, elements from Group VIIIA in the form of charged, energetic particles are directed into the above substrate metals, metal alloys and/or oxides as impurities.
Referring to
The substrates 10 first are introduced into the implantation chamber 14 and held by the holders 26 supported on the base plate 26 of the base 22 of the fixture 20. The next step involves the creation of a proper vacuum environment within the implantation chamber 14. To this end, a vacuum within the implantation chamber 14 must be created which is less than about 5×10−4 torr, and preferably is about 1×10−7 torr. After a plurality of substrates 10 is introduced into the chamber 14, the proper vacuum is established therein 14 by vacuum pump 30. Preferably, the vacuum pump 30 should be of an oil-free type so as to avoid the possibility of introducing surface contamination onto the part to be ion implanted.
With the substrates 10 secured in the fixtures 20 within the implantation chamber 14, the outer surfaces containing the native oxide layer 12 are exposed to a direct line of the incoming ion beam 28. In order to achieve such a direct line, the fixture 20 is rotatable on its base 22 by motors (not shown), as indicated by an arrow 32. The ion beam 28 is of at least one of the elemental doping materials that have been introduced into an ion production chamber 34. The thusly-created ions pass through an ion extraction pre-accelerator 36, then to separating magnets 38 before traveling through an ion accelerator 40. Careful attention must be paid to having the proper ion beam power density acting on the surfaces of the substrates 10. For, if it is too high in certain areas, localized heating may occur. Consequently, the peak ion beam power density of a 50 keV beam should not exceed about ten microamperes per square centimeter.
Control of the ion beam power density can be achieved in a number of ways. Preferably, and as herein illustrated, this low power ion beam current density is affected by expanding the spot size of the incoming ion beam 28 by a magnetic Quadra pole or an electro-static lens system 42. The surfaces of the substrates 10, now secured in the fixture 20 within the implant chamber 14, are then exposed to the incoming ion beam 28, properly modified, if need be, by the lens system 42, for a period from about one minute to about one hour, and preferably for about five to ten minutes. A preferred ion beam particle energy is about 50 keV, and implants a dose of about 1×1014 ions/cm2 to about 1×1021 ions/cm2, preferably about 3×1017 ions/cm2, to a depth of about 100 to 10,000 angstroms, preferably about 200 angstroms, below the outer surfaces of the substrates 10.
The result is that the ion implanted native oxide 12 surface of each substrate 10 has had its properties modified, i.e., rendered electrically conductive, without leaving any discrete layer on its surface. Since the ion implantation process is typically carried out at room temperature, it does not create toxic or hazardous waste, and is readily compatible with clean room and high technology processing environments.
Doping by means of the ion implantation method of the present invention converts the electrically insulating native oxide layer 12 to a doped layer 44 on substrate 10, as shown in
The substrate shown in
The present invention is illustrated further by the following examples.
A tantalum substrate similar to substrate 10 shown in
The substrate was now ready to have the native oxide layer at its outer surface doped with palladium. This was done by first loading the substrate into an ion beam generating apparatus evacuated to a pressure of about 1×10−5 torr. Palladium ions were introduced into the ion production chamber. Then, an energetic ion beam of Pd ions with an energy level of about 50 keV bombarded the deposited surface causing the palladium to mix with the substrate. This continues until the palladium ions had penetrated to a depth of about 200 angstroms at a concentration of about 3×1017 ions/cm2. On completion, the substrate surface was ready to be coated with ruthenium oxide for use in making a capacitor electrode.
A tantalum substrate was prepared according to the previously discussed U.S. Pat. No. 5,098,485 to Evans. In particular, a brush made from palladium wires was brought into contact with and brushed across a dry tantalum surface. It is believed that palladium atoms were thereby mechanically transferred to the tantalum and tantalum oxide. The brushed tantalum was heated to 900° C. for 15 minutes, producing an electrically conductive surface that was then coated with ruthenium oxide in a similar manner as in Example I.
It is therefore apparent that the present invention accomplishes its intended objects. While embodiments of the present invention have been described in detail, that is for the purpose of illustration, not limitation.
